The present invention relates to a method for determining depth values of a body of water using a fan depth finder of the type which emits ultrasonic pulses into the water, receives pulses which have been scattered or reflected by the floor of the body of water for a number of receiving directions, measures the travel times of the pulses, and determines respective depth values from the travel times.
Fan depth finders are used on research and survey vessels to map and explore bodies of water. An angular sector of up to 150.degree. is evaluated and simultaneously measured by a fan depth finder, which is disposed beneath the vessel. Ultrasonic pulses are radiated in this angular sector and received by a transducer with a fan-shaped directional response in a number of individual, narrow sectors. The receiving transducer has tightly-bundled receiving directions. Because most of the receiving directions are oriented diagonally downward rather than vertically downward, some of the pulses that have been scattered or reflected at pulse-impingement or impact points on the floor of the body of water do not propagate linearly to the receiving transducer but instead travel to it along bent paths due to sound refraction. The cause of the bent sound propagation paths is the sound refraction caused by different sound velocity layers in the water. Hence, the actual impact point of the sound may deviate considerably from the apparent impact point because the bend cannot be determined linearly from the angle of incidence of the sound measured in the immediate vicinity of the receiving transducer. By also detecting the sound velocity in the different layers of the body of water, using (for example) a sound measuring probe lowered from the vessel, an average sound velocity can be determined that would result for linear propagation of a pulse from the impact points on the floor of the body of water to the transducer. A measuring procedure of this type for the average sound velocity is very costly for successive measurements of floor profiles of a body of water, because the vessel must slow its travel, if necessary to a dead stop.